Question

The blood of lobsters contains a blue copper porphyrin complex. As a model for the lobster blood pigment, draw the structure of the \(\mathrm{Cu}^{2+}\) pigment of octamethylporphin. What charge, if any, does this complex have?

Short answer

The structure of the \(\mathrm{Cu}^{2+}\) pigment of octamethylporphin has a central copper ion (Cu²⁺) surrounded by a cyclic porphyrin ring with alternating single and double bonds and 8 methyl groups (-CH3) at the outer corners of the molecule. The copper ion binds to all four nitrogen atoms in the porphyrin ring. The entire complex carries a total charge of +2, as the octamethylporphin molecule is neutral and the central copper ion has a 2+ charge.

Step-by-step solution

Draw the structure of octamethylporphin

Octamethylporphin is a cyclic structure with alternating single and double bonds. It has 8 methyl groups attached in place of the 8 hydrogen atoms present in the basic porphyrin ring. To draw iteratively this structure, start with a rectangular planar molecule with four nitrogens in the middle. Each nitrogen atom binds with two carbons creating alternating double bonds. Then draw the 8 methyl groups (-CH3) at the outer corners of the molecule.

Place central Cu2+ ion

In the middle of the molecule, place the central metal ion that is, Cu2+ ion which binds to all four nitrogen atoms.

Determine the total charge of the complex

The copper ion in the center has a charge of 2+. The rest of the molecule, octamethylporphin, is a neutral compound, hence carries no charge. Therefore, the overall charge on the entire complex will be the same as the charge on the copper ion. Therefore, the total charge of the Cu-octamethylporphin complex is +2.

Key concepts

Understanding Octamethylporphin Structure

Octamethylporphin serves as an excellent model for studying a variety of metalloporphyrins, which play critical roles in biological systems. At its core, the octamethylporphin structure is a large, cyclic compound commonly referred to as a macrocycle. This structure is characterized by a ring consisting of four pyrrole subunits interconnected via methine bridges, denoted as '-CH=' units.

In octamethylporphin, there are eight methyl groups (-CH3) that are attached to the pyrrole units. This substitution provides the compound with its 'octamethyl' designation. The architecture of octamethylporphin sets the stage for metal ion coordination due to the presence of four nitrogen atoms in the pyrrole subunits, which act as potential binding sites to a central metal ion.

An excellent method to visualize the octamethylporphin is to think of it as a flat rectangular molecule with a nitrogen atom at each corner. This configuration allows the nitrogen atoms to coordinate with a central metal ion, such as a copper ion, leading to a stable complex.

The Role of Copper Ion Cu2+

Copper ions, particularly the Cu2+ oxidation state, are quite versatile in biological chemistry. The Cu2+ ion can form a strong coordination complex with porphyrins due to its tendency to accept electron pairs from the nitrogen atoms of the organic ligand.

This complexation is of considerable interest because of its resemblance to the active sites of many metalloproteins, including hemocyanin in lobsters, which is a cousin to the hemoglobin found in human blood. The Cu2+ ion, with its d9 electron configuration, fits snugly into the center of the octamethylporphin ring, forming a complex where the metal ion adopts a square planar geometry. Each of the four nitrogens donates a pair of electrons to the Cu2+ ion, stabilizing it within the porphyrin framework and giving rise to its characteristic coordination chemistry.

Total Charge of the Complex

When considering the total charge of a coordination complex, it's essential to sum up the charges on the central metal ion and the surrounding ligands. In the case of a copper porphyrin complex, like the Cu-octamethylporphin complex that is used to mimic the lobster blood pigment, the analysis is straightforward thanks to the octamethylporphin structure being neutral.

The Cu2+ ion carries a charge of +2. Since the octamethylporphin is neutral and does not contribute any additional charge, the total charge of the complex is merely the sum of these components. This means the entire complex has a charge of +2. This positive charge is critical for the complex's interaction with its environment, such as binding with other molecules or participating in electron transfer processes within biological systems.